In principle, for a multiple access communication in a radio channel there are three kinds of methods: the FDMA, the TDMA and the CDMA multiple access. The CDMA method has been made possible by the development of the spread spectrum technique. The spread spectrum technique has many known advantages as compared with the traditional communication methods. One of the fundamental reasons for using CDMA in civil communication (e.g., in mobile cellular network) is the frequency efficiency.
As communication needs have increased there has been a corresponding increase in the number of users which is a problem in mobile systems. The construction of totally new systems and the infrastructures required by them (base stations, exchanges, etc) to overcome these problems is expensive, thus one alternative has been proposed which is to widen the capacities of the existing systems by some additional characteristics. It is, for instance, possible to design "on"- or "in"- the digital TDMA systems (as GSM and DECT) to introduce some CDMA characteristics, whereby the number of the users of the TDMA system can be increased to a total capacity that is possible to reach by the CDMA systems. Thus the system would be a combination of all the multiple access methods, its implementation would be carried out through evolution and its infrastructure would be less expensive.
In the following, the basic factors connected with the CDMA are discussed. In the CDMA system, every user has a specific spreading code by which the users are separated from each other. In an ideal case, the spreading codes are orthogonal in relation to each other. It is thus possible to demodulate from the received signal the symbols transmitted by every user by matched filters according to the spreading codes. In the ideal case when the codes are completely orthogonal, samples taken from the output of the filters include only data from the signal which is to be detected, and is owned by the desired user. In an ideal system it is completely linear and synchronous and where the spreading codes are orthogonal, between the different multiple access methods (FDMA, TDMA, CDMA) there are no differences, as far as the spectrum efficiency is concerned. In practice, however, no system is completely ideal in all conditions. This is why the choice of the multiple access method is significant. For instance, in the mobile radio channel the CDMA system has many advantages: there is less intersymbol interference (ISI) between the symbols than in the narrow band systems, fading will decrease and a narrow band system can operate within the same frequency range as an overlay system with a CDMA system. Also the above mentioned spectrum efficiency or, using another interpretation, the channel capacity will be enhanced.
In practice, in the CDMA system the signals from different users propagate from the transmitter to the receiver along a distance that is different for every signal. Thereby the signals will be delayed in the channel by delays of different length and reach the receiver during different code phases. Furthermore, different users transmit independently from each other which, in addition to the channel influence, causes phase randomness in the received signal. Due to the fact that the orthogonality of the spreading codes is (in general) not possible for all the values of the delays, some influence of the cross-correlations between the signals of other users is seen, in addition to the autocorrelation function of the desired signal, in the outputs of the matched filters to the spreading codes. Thus different users interfere with each other. The created interferences are called multiple access interferences.
In cellular mobile communication, different mobile phones transmit their signals independently from each other, and the signals propagate via different paths to the base station. From the base station the signals to be transmitted to different mobile phones can, however, be transmitted simultaneously. The interfering signals, transmitted by the base station and received by each mobile phone, also propagate via the same path as the desired signal. Therefore, the multiple access interferences are only particularly significant when detecting signals transmitted by different mobile phones at the base station.
The non-orthogonality of the spreading codes is a basic problem that weakens the performance of a practical CDMA system. The different propagation path lengths of the signals also cause differences in the received power levels. This situation arises e.g., when several mobile phones transmit their signals to one and the same base station: the signal, from the mobile phone which is nearest to the base station is stronger than signals from more distant locations when received at the base station. A strong signal disturbs detection of weaker signals. The problem is known as the near-far-problem. In mobile phone telecommunication the near-far-problem is significant particularly when detecting signals from different mobile phones at a base station.
In the CDMA system there would be no near-far-problem, or multiple access disturbances generally, if the spreading codes were accurately orthogonal. Therefore, attempts have been made to solve the problem by generating code families with as little cross-correlation as possible between code members. The multiple access disturbances in the CDMA system cannot in all situations be eliminated through code design, they can only be reduced; the exception being a system with known delays.
When the difference between the power levels of two signals is large, even a minor cross-correlation between the desired and the interfering signal causes a major interference in the detection of a weak signal. This problem can be alleviated by lowering the transmission level of the strong interfering signal so that the received power levels of the desired and the interfering signals are of the same order. This method is called power control. Power control is a straight forward, conventional way to decrease multiple access interferences. The problem in such power control techniques is that the method measures average transmission powers, leading to a worse signal-to-noise ratio and to an increase in the bit error rate. Thus, power control may result in sufficient degradation of a signal so as to annul its beneficial influence.
The interferences created by users between each other can also be decreased by signal detection methods. These methods are called multiple access interference cancelling methods. When there is only one user in the spread spectrum system, or when the spreading codes in a CDMA system are orthogonal, the received signals in a noisy channel can be detected in relation to the bit error probability in an optimal way, using matched filters to the spreading codes. When the spreading codes are non-orthogonal there are better detection methods than the one mentioned above. In CDMA system signal detection studies it has long been assumed that multiple access interferences could be accurately approximated as Gaussian random processes, and that therefore the matched filter and the memoryless detector of the spreading codes could form an optimal receiver in an ideal channel. However, this assumption is not valid in a practical situation.
In practice, a non-ideal cross-correlation function between the codes of different users is detrimental to the operation of the individual receivers as well. It makes both acquisition and tracking of the code synchronisation of the receiver difficult. Respectively, the deviation of the code autocorrelation function from the ideal impulse- like autocorrelation causes problems in code acquisition and tracking, and in a fading multipath channel, in particular.
Therefore there are no spreading codes to create ideal characteristics in all practical situations. By an ideal characteristic is meant the impulse-like characteristic of the code autocorrelation function, whereby it has the top level in relation to length at zero delay value and its sidelobes are zero at all other delay values. Regarding the CDMA multiple use, it is required that the cross-correlation function between the codes in a code set is zero at all possible delay values (completely orthogonal codes) and not only at zero delay.
There is a set of spreading codes with which desired ideal features can be obtained by certain conditions. This kind of a code set comprises complementary codes or sequences. These are characterized by the fact that one single code in the code set is formed by several (equally long) members, the sum of the autocorrelation functions of which is ideally impulse-like. Each CDMA user code is built by the members of the code set so that the sum of the mutual cross-correlation functions is zero at all delay values (non-interacting or completely orthogonal codes or code sets). The formation of spreading codes has been described, e.g., in B.P. Schweitzer, Generalized Complementary code Sets, 1971, doctor's dissertation, University of California, Los Angeles, USA, 87 pages.
When putting into practice a communication system where the number of users is K, K complete orthogonal codes are required. They are formed by K members of a code set, e.g., in the way presented in Schweitzer's study. The transmission of the members of the codes of each user requires K orthogonal channels where, by combining (summing) the output signals of the matched filters a situation in a multiple access case is reached where the users do not interfere with each other. For instance in FIG. 1 there is a case of two users (K=2).
Depending on the transmission channel, the orthogonal channels are created in many ways. If the coherence time of the channel is long enough, one transmission channel can be split into K different time slots and thus create K orthogonal channels. Respectively, if the width of the coherence band of the transmission channel is wide enough, the channel can be split into K non-overlapping frequency intervals whereby the orthogonal channels are created. Other orthogonalization possibilities with one channel transmission comprise, e.g., the use of the quadrature components of the carrier wave (K=2) and the use of different polarisation levels or rotation directions.
FIG. 2 shows a two user CDMA system illustrating the principles of the application of a complementary code set e.g., in a cellular network. A cell includes two mobiles MS#1 and MS#2 and a base station BS, comprising the receivers for both of the users #1 and #2. Ideal characteristics of the complementary code set are brought in to use when the radio channel is time-multiplexed into two orthogonal channels 1 and 2, completely independent from each other. The assumption thereby is that the coherence time T.sub.D is long enough, regarding the orthogonal signal processing required. In a two user system two codes (a code pair) for each user are required. Interferences and noise will be summed into the signals in the radio channels. In this example, it is assumed that the radio channels are ideal. The spreading codes of the users #1 and #2 are e.g., selected according to the principles presented in the Schweitzer study, so that they will, at all delay values, form a complete orthogonal code pair. The spreading code waveforms of the data signal d.sub.1 of mobile MS#1 are s.sub.11 for channel 1 and s.sub.12 for the orthogonal channel 2. In the base station receiver #1 there is a filter h.sub.11 matched for the code of channel 1, and h.sub.12 for channel 2. As a principle, by summing the output signals R.sub.11 and R.sub.12 of the filters an ideal detection result d.sub.1 for the user #1 is derived from the summed signal R.sub.1. Respectively, the spreading codes of the mobile MS#2 are s.sub.21 and s.sub.22 and the matched filters h.sub.21 and h.sub.22, the summed signal R.sub.2 =R.sub.21 +R.sub.22 of which gives an ideal detection result for the user #2 (d.sub.2).
The signals of the base station receivers are discussed at a principle level. It is assumed that the data signals have been BPSK spreading modulated to the carrier wave and the bits are presented as follows: bit "1".fwdarw. waveform "+" and bit "0".fwdarw. waveform "-". As an example, let us assume that the system (K=2) uses the following signals (a complementary code pair and to it a complete orthogonal pair): the spreading codes s.sub.11 =+++-, s.sub.12 =++-+ of MS#1, the waveforms s.sub.21 =+-++, s.sub.22 =+--- of MS#2. Thus, the impulse responses of the matched filters of the base station BS are: receiver #1 h.sub.11 =-+++ and h.sub.12 =+-++ and receiver #2 h.sub.21 =++-+ and h.sub.22 =---+. It is assumed in this example that the data signals of the users are binary (d.sub.1=" 1", d.sub.2 ="0") and that all gains are normalized as ones. Then MS#1 transmits the waveform s.sub.11 =+++- to channel 1 and the waveform s.sub.12 =++-+ to channel 2. Respectively, MS#2 transmits the waveforms -s.sub.21 =-+-- and - s.sub.22 =-+++, when the influence of the data bit d.sub.2 on the BPSK signal is taken into consideration.
FIG. 3a shows signals of the base station receivers when no interferences nor any noise is summed to the transmission signals in the channels, and when all the gains are assumed to be ones. The receiver gives to both of the users #1 and #2 a waveform that resembles an ideal impulse function and from which the required data symbol detection can be made. FIG. 3b illustrates a multiple access situation when the spreading signals of user #1 (MS#1) are also arriving at the receiver of user #2 (MS#2) and vice versa. The aim of this figure is to show that no multiple access interferences or non-interacting arise and no power control are needed.